Hitherto, as a charge generating material for an electrophotographic photoreceptor, various materials are known and organic pigments are also used. The characteristics of a charge generating material give a large influence on the characteristics of the electrophotographic photoreceptor. In the case of using an organic pigment as a charge generating material, the structure itself of the organic pigment largely influences on the characteristics of the photoreceptor.
It has been found by the inventors that metal impurities contained in the organic pigment also give influences on the characteristics of the photoreceptor. Such metal impurities sometimes enter an organic pigment during the production of the pigment. Examples of the metal impurity entering in the step of producing the organic pigment are metals from a reaction vessel when the vessel is made of stainless steel, iron, etc., from a container in which the pigment is ground, or from residues such as reaction reagents, a catalyst, etc., impurities formed by the corrosion of a metal vessel in the case of carrying out acid pasting in the metal vessel or impurities by the corrosion of the vessel by a halogen gas. As the metal impurities entering the organic pigment, iron is most and according to the entering route, iron oxides are also included. Other metal impurities entering an organic pigment are stainless steel components such as Cu, Ni, Cr, etc., transition metals such as Zn, Al, Mg, Ti, V, Co, W, etc., and an alkali metal such as K, Ca, Na, etc.
Many of these metals have an electric resistance lower than the electric resistance of pigments in the dark and in particular, when such a metal exists in the pigment in the form of metal oxides, metal sulfides, metal chlorides, etc., the electric resistance of the pigment is lowered so that the resulting photoreceptor tends to be undeveloped in portions. Also, if such metal impurities exist, the chargeability of the photoreceptor is frequently lowered wholly.
Furthermore, even when the light-sensitive layer is formed using the same charge generating material, the resulting photoreceptors sometimes exhibit different chargeabilities or dark decay, and hence there are problems that the photoreceptors having uniform electrophotographic characteristics are produced with difficulty, and those having a low chargeability and a high dark decay are often obtained.